In To Kill a Mockingbird, in Scout's eyes, what is Atticus's chief fault?
As any typical child who puts her father on a pedestal, Scout is as loving as she is judgmental about her Dad. The key criticism she shares in chapter 10 is that Atticus is "feeble", or a weakling, in comparison to other dads. Keep in mind that Scout does not take into consideration that the Finches come from a more educated, sophisticated and civilized family than most of Maycomb. This, and the fact that Atticus was a respected man of law and politics, renders him mindful of his behavior, always ringing true to the persona that everyone knows, admires, and trusts.
To Scout this is just part of his boring personality.
Atticus was feeble: he was nearly fifty. When Jem and I asked him why he was so old, he said he got started late, which we felt reflected upon his abilities and manliness.
(Ouch!) This feebleness, according to Scout, is the reason why he does not play football with Jem, nor hunts, nor farms, nor drives a truck, nor goes fishing, nor shoots (they will find the truth about that later).
Moreover, he also wears glasses as he is nearly blind in his left eye--a genetic pre-disposition in the family, according to Atticus. Therefore, he has to move his face in a funny way to be able to see things with his good eye.
None of these things make Atticus stand out in a cool way. All he does is "stay in the living room and read". Imaginable for a kid, Scout feels that these traits are what very dull people are made of. It is understandable that Scout describes her father this way, even in retrospect.